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Chapter 5 Single Gene Inheritance
Inheritance of Single genes
Equal segregation of alleles during meiosis
In meiosis, each of the four haploid products receives one of each kind of
chromosome
A/A homozygotes  all get A chromosomes
A/a heterozygotes  half get A chromosomes-half get a chromosomes
So…. alleles of heterozygotes segregate equally
Interpretations of Inhertance
Haploid organisms, fungal spores haploid mating: A  a
Easy with some fungi and protists bcause all four haploid products of the meiocyte
(tetrad) can be analyzed
Grow up as haploid
Spores remain in order
Diploid organisms diploid mating: A/a  A/a
-Usually nonordered random products
-Usually to interpret segregation after fused
and diploid again.
-More possibilities with diploid organisms
Three possible diploid genotypes
A/A a/a
A/a
Six possible pairs (crosses)
Three potential outcomes, 4:0, 1:1, 3:1
Begins with pure breeding lines
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P1(green)  P2(yellow) parental generation

F1 (yellow)x F1 (yellow) first filial generation

F2 3 yellow:1 green second filial generation
green disappears in F1
but reappears in the F2
Indicates yellow is dominant
3:1 ratios are typical in monohybrid crosses
indicates the F1 must have been heterozygous
Test cross: uses homozygous recessive to distinguish between AA and Aa
Sex-linked genes have different rules
Sperm
Eggs Gametes 50% X 50% Y
50% X
XX
XY
50% X
XX
XY
An

mating yields a 3A:1a phenotypic ratio, but the recessive phenotype
is restricted to 50% of male offspring.
Sperm
Eggs Gametes 50% XA 50% Y
50% XA XAXA XAY
50% Xa XAXa XaY
XAXa
XAY
Pedigree Analysis
Rare Autosomal Recessive Disorders
males are square
females round
Two children, one of each sex, show the trait
Conclusions: must be autosomal recessive trait,
parents must be heterozygous,
2/3 chance that each unafflicted child is heterozygous
examples: PKU, Tay-Sachs, albinism
Rare Autosomal Dominant Disorders
Same as above if one parent was affected
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Example Huntington disease
Common Polymorphisms
Likely interpretations change is NOT rare (Marriage often brings in carrier)
Rare Sex-Linked Disorders
If gene for trait is rare,
most consistent with X-linked recessive
*just one affected female would indicate autosomal
Risk Assessment
The product rule
1st : looks like autosomal recessive
2nd assumed its rare.
(Walk through)
The probability that each grandmother is
heterozygous is 2/3; the probability that each
parent of the offspring is heterozygous is x ½. If
each is heterozygous, the probability of an afflicted
child is ¼. Therefore, using the product rule, the
probability that the child will be afflicted is x ½ x
x ½ x ¼ = 1/36
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Maternal Inheritance of Organelles
Mitochondria and chloroplasts. small number of genes on circular chromosome.
mostly inherited through maternal lineage via egg cytoplasm